Physics of Reversed Field Pinch Relaxation

Physics of Reversed Field Pinch Relaxation Susanna Cappello Consiglio Nazionale delle Ricerche - Consorzio RFX - Italy susanna.cappello@igi.cnr.it The Reversed Field Pinch configuration is characterized by relaxation processes which lead to self-organized helical states. This is observed both in experiments at high plasma current [1, 2] and in 3D nonlinear MHD simulations [3, 4]. In experiments, quasi-helical regimes feature clear internal electron transport barriers [5], which are intermittently interrupted by reconnection events and development of stochastic magnetic topology in the plasma core [6]. 3D nonlinear MHD numerical modeling can reproduce with high fidelity the macroscopic dynamics of high current RFP experiments [4]. Moreover, seed helical magnetic perturbations (MP) at the magnetic boundary are predicted to mitigate the intermittency up to producing steady helical states, and to be able to stimulate alternative helicities. Of particular interest is the possibility to stimulate non-resonant helical states, which feature improved topological properties [7]. The formation of "hidden" confining structures, accompanying the process of magnetic chaos healing, is proposed to be at the basis of the electronic transport barrier formation [8, 9]. First experiments in RFX-mod appear to confirm this prediction. Keywords: Magnetic Relaxation, Magnetic Chaos Healing [1] D. F. Escande et al., Physical Review Letters, 85, 1662-1665, (2000) [2] J. S. Sarff et al., Nuclear Fusion 53, 104017, (2013) [3] S. Cappello and Paccagnella, Phys. FluidsB 4, 611-618, (1992) [4] D. Bonfiglio et al., Physical Review Letters, 111, 085002, (2013) [5] R. Lorenzini et al., Nature Physics, 5, 570-574, (2009) [6] M. Gobbin et al., Plasma Phys. Control. Fusion, 57, 095004, (2013) [7] M. Veranda et al., Plasma Phys. Control. Fusion, 55, 074015, (2013) [8] S. Shadden, F. Lekien and J. Marsden Physica D, 271, 212 (2005) [9] G. Rubino, Borgogno, Veranda et al., Plasma Phys. Control. Fusion, 57, 085004, (2015)